i soldered at my max 480C, worked great.
Is there a specific watt iron I should buy?
I only have a 48 watt iron is that gonna be powerful enough it goes all the way to 450 max? And is 62/36/2 rosin core silver bearing solder ok?
Iāve read a lot of people around here advising not to use soldering irons on 18650 batteries because the heat damages them, spot welding seems to be the common practice to build battery packs.
I went and started my build with a soldering iron and its going pretty good, 3-4 seconds on each cell to bond the nickel strip.
But how would you test if there was any damage done to the soldered cells? Would there be a way to test them?
Test voltage is what I do when I solder mine , I also bench test for 2 days before riding but thatās because Iām paranoid
The only way to tell is by measuring capacity. Get 200W soldering iron and do it super quickly.
I did NOT read the rest of the thread, but you can find guides to build a small capacitor based spotwelder for cheap, it is better for the cells, as soldering destroys some of the internal of the cells
Saw a video of a person rolling a soldered and spot welded battery out, the soldered was discolored on the inside almost 5-8mm down 
Dont know the impact of the life, but discoloring is normally not a good thing
I solder them all the time. Infact I would be so bold as to suggest that soldering is a much better connection than miniscule spot welds.
However an inexperienced solderer may have apply too much heat to the battery and thats bad.
A skilled solderer should be able to solder a battery tab in seconds.
I used between 4 and 6 seconds of solder time on each pole, the voltage everytime was exactly as it started, the solder was noticeably better than the spot welding I was doing before, you can not tell if the spot weld is good enough by just looking at it, you need to pull the nickel to strip it off with pliers and see if it holds.
So is there another way to test the battery capacity to see if there has been any damage besides measuring the voltage? Or people says itās bad without scientifical proof?
I just soldered a 10s4p a week ago, 90w iron, was really quick. Made the mistake to be abit rushed on one of the paralells and soldered another battery to the same copper braid abit to early, and copper leads the heat very fast.
Battery didnt feel to hot but I can almost say it mustāve gotten really hot, it survived for now, will see how it will hold up with time 
but thats one thing to think about I guess,else it was pretty simple aslong as the surface was ruffed up abit.
Worth to mention Im rusty at soldering as I have not done it all that much for the last couple years.
Im also not sure if the damage it would make shows in long term effects or straight onto the voltage as you work, heard abit of both but never anything to convince me whats correct or not.
If youāre using old laptop batteries like me, it doesnāt matter, these old batteries have been over heated many times, a couple seconds with the iron is not gonna do permanent damage.
Hay guys Iām just gonna add this in here for the people thinking of soldering your batters togetherā¦its not safeā¦if they get to hot they will fail and I donāt think you want to be half way into a build and the batter your on goās south. I do have an alternative optionā¦in the world of Arduino there are portable spot welders that you can use. they use a small lipo battery to charge but its much better and gives you more room then a desktop spot welder. here is the link to one Iām buying here soon . this will save you guys the trouble of having to buy a spot welder and a solder at the same time. I live in an apartment and its oldā¦so I donāt think I can use a normal spot welder. I fear ill start a fire or something.
Soldering onto the cells could damage safety vents and the CID (Current Interrupt Device). You wonāt know that you damaged these until itās too late and the cell could fail in a very dangerous manner, such as an explosion or runway thermal event. Unless you are a very expert solderer and have the right equipment, I wouldnāt attempt it. In fact, since itās impossible to tell if you damaged these things, I would just avoid soldering altogether.